In the gatch bed having an air mattress, a back bottom 1a, hip bottom 1b, buttock bottom 1c, thigh bottom 1d, knee bottom 1e, leg bottom 1f are arranged in the longitudinal direction of the bed, as shown in FIG. 1. Of these bottoms, the back bottom 1a, thigh bottom 1d and leg bottom 1d, for example, can be adapted to move up and down to thereby raise the back of the bed user (a person lying on the bed) for easy reading and other purposes or lower the back and flatten the bed for easy sleeping, easy medical examination and other purposes.
Placed on these bottoms 1a to 1f is an air mattress 2. As shown in FIG. 2, this air mattress 2 is formed of a plurality of air cells 3 extending to the bed width direction and arranged in parallel in the bed longitudinal direction. Patent Document 1 discloses a gatch bed using this air mattress. Patent Document 2 discloses a gatch bed using a mattress having nonwoven fabric as its core.
Of those gatch beds, when the back bottom 1a is raised in a back raising operation, the undersurface of air mattress 2 in contact with the back bottom 1a and others and the top surface of air mattress 2 in contact with the bed user become different in length in the longitudinal direction of the bed. That is, in the process of the back bottom 1a being moved up from the horizontal position (tilted upward), the top surface of air mattress 2 that is in contact with the bed user becomes shrunken while the length of the undersurface of air mattress 2 will not change because it is in contact with the back bottom 1a and others.
Particularly, in the case of air mattress 2, there is a mode in which a plurality of air cells 3 are successively and repeatedly inflated and deflated to thereby prevent the bed user front getting bedsores on their skin. If a back-raising operation is actuated while this mode is ongoing, the pressure of some of air cells 3 is lowered because the pressure of air cells is repeatedly increased and decreased successively. Such air cells 3 with their pressure lowered become likely to shrink due to the bed user's weight. Further, the air cells 3 of high pressure tend to fail down due to the bed user's weight if the pressure of the adjacent air cells is low. In this way, when the back section of air mattress 2 is moved up for a back raising operation, the top surface of air mattress 2 contracts with respect to the bed longitudinal direction, and the total length of air mattress 2 becomes shorter.